U.S. Pat. No. 7,469,092, describes a method and apparatus for the monitoring and control of a process using tunable diode laser absorption spectroscopy (TDLAS). Briefly stated, the TDLAS method and apparatus involves directing a beam of light, which may be a multiplexed beam of a number of distinct wavelengths, into a boiler or furnace combustion chamber to measure boiler or furnace combustion properties such as temperature and the concentration of various combustion species including CO, CO2, O2 and H2O. TDLAS monitoring techniques are based on a predetermined relationship between the quantity and nature of laser light received by a detector after the light has been transmitted through a region of interest and absorbed in specific spectral bands which are characteristic of the gas species resulting from combustion. The absorption spectrum received by the detector may be used to determine the quantity of a gas species under analysis plus associated combustion parameters such as temperature.
The technique requires a line of sight through the boiler or furnace. In fact, many lines of sight are typically required as it is often desirable to measure combustion properties in multiple boiler or furnace locations. Typically a wavelength multiplexed laser beam is transmitted from a pitch optic to a catch optic on the opposite side of the boiler or furnace. Certain applications require up to 15 or more measurement paths, thus requiring 15 or more pitch/catch optic pairs and 30 or more furnace penetrations. However, the use of 15 or more pairs of substantially identical pitch/catch optics and the need for 30 or more corresponding furnace penetrations imposes high costs, not to mention increasing the complexities of the system. In some cases, installation of the system may require waiting years for the scheduled shut-down of the boiler or furnace.
Because of the costs and complexities of the optics and corresponding furnace penetrations, the practicality of the conventional system is limited in the case where one desires to monitor (and perhaps also control) a larger number of combustion zones within the furnace.
The present invention is directed toward overcoming one or more of the problems discussed above.